One of the major problems which affects present-day power systems is that they are designed for efficient operation only at fundamental or conventional power system frequencies. When other power equipment is used at or nearby a particular power system, it is quite common that frequencies generated by the equipment act to alter the currents and voltages of the power system so that a disruption of the power produced by the system can occur. Experts in the electric power industry have long studied the effects on electric power production caused by particular equipment in a continuing effort to monitor and alleviate such problems.
A particularly severe problem in this area is caused by the increased use of equipment that can generate harmonic currents and subsequent harmonic voltages in electric power systems. Harmonic generating equipment such as DC links, static VAR (volt-ampere reactive) compensators and large variable speed motor controllers are now commonly used in power systems. The systems which employ this equipment have been and will continue to be degraded by the effect of harmonic current generation so that the quality of electric power gradually decreases. A major cause for this substantial disruption is that the amount of harmonic current necessary to disturb the normal operation of electric power systems can sometimes be very small, e.g., less than 1% of the system, when compared to the full-load rating.
Another aspect of this problem is that harmonic current injected at one point on a power system can create disturbances hundreds of miles away from the source. Consequently, the damage caused to particular components of a power system when exposed to these harmonic currents can have extremely deleterious long-range results. Industry research into this problem has identified a number of cases where harmonic generation by one piece of equipment can cause a decrease in performance or more severe damage to another part of the electrical power system. For example, it has been determined that excessive 5th and 7th harmonic current present in the windings of a large steam-driven generator can combine to produce a potentially damaging 6th harmonic vibration in the turbine. Clearly, the need exists for a relay that can monitor and provide a protective response to low-level harmonic currents which are produced in electric power systems, particularly in situations where more than one harmonic current may be produced.
In the patent art, it has been known to provide various protective relays for power systems. For example, U.S. Pat. No. 4,513,344 (Usui) discloses a relay for detecting faults in a power system that includes means for extracting a fundamental wave component and a second harmonic component from a differential signal. In U.S. Pat. No. 3,579,040 (Tennenbaum) there is disclosed a protective relaying system in which the magnitude of the second harmonic component can trigger a set of restraint elements. Another patent along these lines is U.S. Pat. No. 4,689,708 (Hager et al) which provides a zone protective relay scheme for monitoring and controlling power lines including means for determining in which direction a fault is occurring. In none of these patents, however, is there disclosed a device which can accurately respond to low-level harmonic overcurrents produced in an electric power system and which can also be used to effectively monitor at least two different harmonic components at the same time.